1. Technical Field
This invention relates generally to coated filtration membranes and more particularly to the improved resistance of such coated membranes to aquatic fouling by aqueous borne contaminants.
2. State of the Art
A common problem encountered in the processing of aqueous media by membrane filters is the reduction of flux due to fouling. Fouling may be caused by one or more materials present in the aqueous media such as dissolved species, colloidal species, suspended matter, surfactants or surfactant-like chemicals, microbiological entities, and the like. The fouling species can be selectively concentrated at, or absorbed to, the active surface of the membrane filter, thereby reducing the effectiveness of the filter. A decrease in flux due to fouling typically results from the deposition of one or more layers of such material on the input surface of the membrane filter. These layers usually thicken over time, reducing the rate of permeate flow such that higher pressures become necessary to maintain the flow rate. This typical mode of operation also requires frequent backflushing and cleaning operations that increase the size of the filtration system, the power required for filtration, and the logistics of installing and maintaining the system.
While fouling in a general sense occurs on a wide variety of surfaces and due to many different environmental cues, it is generally presumed that fouling by water-borne species is promoted or augmented by the presence of hydrophobic sites on a filter. Hydrophobic sites on membranes can, for example, act as accumulation sites for organic contaminants in water and initiation sites for the colonization and development of biofilms. Membrane filters used to process aqueous media are commonly prepared from hydrophobic polymers such as polysulfone and are therefore quite susceptible to fouling, particularly by biological materials or products such as proteinaceous species. In addition to affecting flux, the size selectivities of hydrophobic membrane filters may be modified by the fouling process and surface pore diameters can be changed by the deposition of adsorbent foulants on the pore walls.
Information relevant to attempts to alleviate such problems by increasing the surface hydrophilicity of a membrane in order to promote fouling resistance can be found in the following references: U.S. Pat. Nos. 4,087,388; 4,845,132; 5,468,390; 6,177,011; 6,913,694; U.S. Patent Application No. 2005/0176893; U.S. Patent Application No. 2010/0159143; U.S. Patent Application No. 2010/0133172; U.S. Patent Application No. 2010/0059433 A1; A. C. Sagle “PEG hydrogels as anti-fouling coatings for reverse osmosis membranes” (2009) Ph.D. Dissertation, The University of Texas at Austin Publ. No. 3369184, 187 pages; and K. Mizoguchi, K. Fukuia, H. Yanagishitab, T. Nakaneb and T. Nakatac “Ultrafiltration behavior of a new type of non-ionic surfactant around the CMC” (2002) Journal of Membrane Science 208 285-288. However, each of these references suffers from one or more of the following disadvantages:
1. Coatings for fouling mitigation reduce the initial flux of water through hydrophilic filter materials due to the additional hydraulic resistance presented by the relatively thick and uneven layer of coating material. This reduction in initial water flux typically cancels out any beneficial reduction in flux loss provided by the coating due to improved fouling resistance;
2. Most coatings are coated on top of membrane layers and, since they are physically weak, do not withstand abrasive feeds such as seawater;
3. Hydrophilic coatings are not expected to withstand chemical clean cycles because they are inherently not as chemically resistant as hydrophobic coatings;
4. A majority of coatings cannot be regenerated in situ in the field; this precludes regeneration of coatings removed through physical abrasion and/or chemical cleaning in a simple and cost effective manner; and
5. Approaches other than coatings also suffer from significant disadvantages. Blended membranes with combined hydrophilic and hydrophobic groups, for example, are more costly to fabricate and more susceptible to damage during chemical cleaning.
Other approaches to reduce fouling typically involve pretreatment of feedwater with potentially hazardous chemicals including, for example, ion precipitation and oxidation by halogenated species such as chlorine and iodine. Such treatment also significantly increases the environmental footprint of the membrane step and costs associated with membrane operation.
Thus there remains a need within the aqueous media processing industry for nonhazardous methods and filters that reduce the rate of fouling and therefore the overall cost of filtration.